This invention generally relates to alignment marks, such as alignment marks for photolithography. More particularly, it relates to alignment marks on masks or reticles, and on semiconductor wafers, for aligning the position of the wafer to the position of the mask or reticle.
Semiconductor wafer fabrication involves building a structure in sequential steps including ion implantation, diffusion, deposition, contact formation, and metalization. Most of these steps require precise positioning of the wafer with the masks, and these masks must be almost perfectly aligned to the structure formed on the wafer by the previous steps. A particular mask is aligned to the wafer with an alignment system included in photolithography equipment that recognizes alignment marks printed on the wafer.
In most currently available commercial align and expose equipment, the mask to wafer alignment is accomplished automatically in several phases: a coarse alignment, an intermediate alignment, and a fine alignment. For each of these phases there may be separate alignment marks on the wafer and separate mark reading and alignment systems in the equipment. Generally, the coarse alignment of the mask and wafer are done without reference to each other. Fine alignment systems on previous generations of equipment depended on manual alignment. An operator located the alignment marks and adjusted the position of the wafer and mask or reticle. When the operator saw that the alignment error was small enough he or she would expose the wafer.
Current alignment systems perform the fine mask and wafer alignments automatically, without operator intervention. An automatic alignment system consists of an alignment mark on the wafer and on the mask, an alignment sensor or measurement system, an algorithm to interpret the alignment measurement data and to calculate misalignment, and a mechanical positioning system to correct the position of the mask and wafer and to hold them in alignment throughout the exposure.
There are presently several different types of aligiment systems, including ones that use bright field, dark field, through the lens illumination, off-axis illumination, wide wavelength range, narrow wavelength range, or single wavelength, and staring or scanning.
The alignment algorithm software confirms that the correct mark has been selected, uses estimation or curve fitting to determine the location of marks and to minimize the effect of measurement error, combines all available data to determine a best estimate of position and alignment errors, calculates correction commands, and transmits those commands to the positioning system.
Alignment marks typically consist of narrow bars or gratings oriented to provide x and y positioning information. The alignment mark is printed in the kerf, the narrow space between chips on the wafer, at an early processing step and that mark is typically read at several subsequent masking steps so each of these subsequent masks is aligned to the same mark. The marks must be robust to survive wafer fabrication steps. They must also be of unique design so that there is no confusion with circuit patterns.
However, a problem arises because the marks are different for nearly each type of photolithography equipment. Different vendors all have different marks. And even for different equipment supplied by the same vendor the marks can be quite different. Thus, if a first level is printed with a first mark, only certain photolithography tools that can read that mark can be used on all subsequent levels. To overcome this problem, two or more alignment marks have been provided on each mask and printed in the kerf that are readable by different photolithography alignment tools so that subsequent levels could be exposed by different exposure equipment. However, these additional marks consume valuable kerf area. Thus, a better solution for alignment is required to provide a way for different equipment to read alignment marks without using up a large amount of kerf area, and solutions are provided by the following invention.
It is therefore an object of the present invention to provide an alignment mark that can be read by different alignment systems.
It is a further object of the present invention to provide an alignment mark that combines elements of alignment marks used for each alignment system.
It is a further object of the present invention to provide an alignment mark that is the combination of alignment marks used for different alignment systems.
It is a feature of one embodiment of the present invention that an alignment mark includes a portion of a first alignment mark for a first system and a portion of a second alignment mark for a second system.
It is an advantage of the present invention that a single alignment mark can be read by two or more different alignment systems.
It is an advantage of the present invention that kerf area is saved by combining alignment marks into a single alignment mark.
These and other objects, features, and advantages of the invention are accomplished by a structure comprising an alignment mark which has a first aspect and a second aspect different from the first aspect. A first photolithography tool is capable of aligning to the first aspect and a second photolithography tool is capable of aligning to the second aspect of the alignment mark. The first photolithography tool is substantially insensitive to the presence of the second aspect and the second photolithography tool is substantially insensitive to the presence of the first aspect.
A second way of describing the invention is a structure comprising an alignment mark which is capable of being read by a first photolithography tool and by a second photolithography tool. The mark comprises horizontal lines or vertical lines, and the first tool reads the mark with light reflected or diffracted from the horizontal or said vertical lines. The mark also comprises at least one diagonal line, and the second tool reads the mark with light reflected or diffracted from the at least one diagonal line.